This invention relates to a cutting apparatus and more particularly, to an apparatus capable of cutting a running web accurately even at high speeds.
Numerous devices have been contrived over the years for cutting running webs of material. Many of these devices have involved, power-driven, counter-rotating opposed drums each provided with co-acting blades which sever the web.
Where constant angular velocity of the rotating components is maintained, it is possible to obtain a relatively uniform cut length of successive sheets. However, mechanical roll drives inevitably have clearance or slack at various point that contribute mechanical backlash. When such blacklash is combined with shaft twisting and other structural deformations, the result is instantaneous velocity differences at the blade tips which, in turn, gives differences in sheet length of as much as .+-. 1 mm. For many applications these differences are excessive. These differences are influenced by web dynamics, e.g., web flutter and the like, which account for part of the length variation. The effect of web dynamics increases when high web speeds of, for example, 50 meters/min. or higher, are attempted.
Apart from these length variations, a problem is often encountered with blade life which in some cases can be so short as to require a readjustment and resharpening of the blades virtually with every change of a factory shift. A still further problem encountered with the cutting apparatus of the prior art is that it is prone to breakdown due to the extreme speeds and forces to which it is subjected. This is particularly true when tough materials such as some of the sheet plastics used in photographic film making are to be cut.
Among the various prior art apparatus that is known and been found unsatisfactory is a rotary shear knife designed by W. W. McFarren. This shear knife is described in U.S. Pat. No. 2,125,939 issued Aug. 9, 1938 and involves two counter-rotating shafts each mounting tapered blades each having an involute shaped cross-section along the length of the blade. This arrangement is subject to all of the problems noted above such as length variation of the cut pieces and, in addition, because of the involute shaped blades, the web cannot be cut at right angles. A final problem is encountered because the involute shape itself causes excessive wear of the blades -- hence the requirement for frequent resharpening. A final problem is due to the dynamics of the cutting operation; the blades themselves tend to scrape along the surface of the film that is cut. This is disadvantageous particularly in the case of highly sensitive photographic films which would tend to become scratched under such rough treatment.
A greatly improved cutter was designed by Shields and described in his U.S. Pat. No. 2,246,957 issued June 24, 1941. Shields does not solve the accurate cutting length problem, but he does bend or shape the blades along their respective lengths such that the cutting edge of one of the blades approximates an epitrochoid curve while the other blade is parallel to the axis. Because of the taper of the blade edge, the actual cut is a progressive shearing action. Unfortunately, despite the significant improvement provided by Shields, the bending of the long blade is at best somewhat inaccurate and requires tedious adjustments to even closely approximate an edge following a true epitrochoid curve. Once this is accomplished, the slightest jar or nick in the edge of the blade will produce a gap and often destroy the epitrochoid curve of the edge. This is totally unsatisfactory particularly if precise cuts are required. Furthermore, once a blade is resharpened the entire alignment of the blade must be readjusted.
Huck describes in his U.S. Pat. No. 2,738,842 issued Mar. 20, 1946, a technique whereby the web to be cut is wrapped around one drum which mounts a straight edge blade parallel to the axis of the drum. This permits accurate lengths to be cut. Furthermore, once the cut is achieved the front edge of the cut is clamped such that it is firmly secured to the drum so that it may more accurately meter the next section of web to be cut as the drum rotates. Despite these improvements, Huck combines an involute shaped blade with a straight edge which creates an inherent mismatch. He makes up to a large extent for this mismatch by the utilization of a spring-loaded blade which co-acts with the involute shaped blade. The spring loading, in and of itself, however, means that extremely tough materials cannot under any conditions be cut with the desired degree of reliability. Furthermore the mechanical clamping arrangement is at best somewhat tedious and prone to breakdown and does not lend itself to high speed operation.
A significant improvement over the mechanical clamping arrangement is taught by Nystrand et al. in their U.S. Pat. No. 3,338,575 issued Aug. 29, 1967. Nystrand et al. describe a valved vacuum retention technique in which the leading edge as well as the trailing edge of the webs are maintained by a plurality of vacuum holes disposed over the periphery of the drum around which the web is wrapped for metering purposes. Trogan, in his U.S. Pat. No. 3,709,077 issued Jan. 9, 1973, improves on the vacuum hold down techniques by providing a slight recess in the periphery of the drum with a vacuum port such that as the cut occurs, the film is tucked into the recess and held there tightly and securely by the vacuum until subsequently released at the proper portion of the web cycle by a suitable vacuum valving arrangement.
Unfortunately, all of this prior art suffers from unreliability, repeated failure and the requirement of the blades be frequently sharpened, each sharpening necessitating a complete readjustment of the alignment of the respective blades. It is an object of this invention, therefore, to obviate many of these disadvantages of the prior art cutting apparatuses.